Editor Skeleton3D CreatePhysicalSkeleton skips bones if multiple children

### Tested versions

* Reproduced in:
- 4.2.1-stable
- 4.3-dev2

### System information

Godot v4.2.1.stable.mono - Windows 10.0.22000 - Vulkan (Forward+) - dedicated AMD Radeon(TM) Graphics () - Intel(R) Core(TM) i5-4690K CPU @ 3.50GHz (4 Threads)

### Issue description

When creating physical bones for a skeleton via the editor, a bone is only created for a single child. Any remaining children are skipped. (See `Skeleton3DEditor::create_physical_skeleton()` where only single parent bone is created for each child)

I also noticed some rotation errors due to target and up vector being parallel, likely because the required cross check in `create_physical_skeleton()` is done on the non-normalised vector. (See `Basis::looking_at()` where the cross check is performed on normalised vector). I was able to work around this by creating my own import script.

I also noticed that selecting a skeleton bone in the property inspector (ie, not the physics bone) seems to select the child bone(s) rather than the named bone. So in the case of a bone with multiple children, all child bones are selected.

Physical Bones with multiple children are missing all but first:

Selecting skeleton bone highlights child bones in main view:

### Steps to reproduce

Load Project
Open Scene

### Minimal reproduction project (MRP)

[BonesTest_4.2.1-stable.zip](https://github.com/godotengine/godot/files/13984410/BonesTest_4.2.1-stable.zip)

Start in Skeleton3DEditor::create_physical_skeleton() and inspect the reported child-bone creation and vector checks, then compare the behavior with Basis::looking_at(). Reproduce the issue using the linked BonesTest_4.2.1-stable.zip project. Done means physical bones are created for every child, rotations avoid the reported errors, and selecting a skeleton bone highlights the named bone rather than its children.

Written by the indexing model from the issue text.